Chemical vapour deposition (CVD) is an established technique for depositing material onto a substrate. The technique has been extensively described in patent and other literature. For deposition of diamond, the CVD process typically involves providing a gas mixture that, on dissociation, can provide carbon and hydrogen. The dissociation of the source gas mixture is brought about by an energy source, such as microwaves, radio frequency energy, a flame, a hot filament or jet based technique. The reactive species are allowed to deposit onto a suitable substrate, typically held at between 700° C. and 1200° C., to form diamond.
The minimisation of the presence of defects within CVD diamond is of utmost importance for several applications. There are different types of defects that occur in CVD diamond. Point defects can occur when impurities in the growth atmosphere are incorporated into the diamond lattice. Another type of defect is a dislocation. Dislocations form within the crystal, possibly due to the formation of pits on the diamond growth surface, and can further multiply during growth. Such pits may also be responsible for the inclusion of other defects and impurities.
The increasing presence of these defects is detrimental to several properties of the CVD diamond material. An increasing presence of all types of defects affects certain properties, for example, decreasing the thermal conductivity (as phonons are scattered). The point defects also affect absorption of photons and are therefore deleterious to optical transparency. Dislocations result in local birefringence due to their anisotropic disruption of the cubic symmetry of the lattice and so are also detrimental to the optical properties of the diamond material.
It has been found that dislocations in homoepitaxial CVD diamond layers tend to nucleate at or near the interface with their substrate. It has also been found that dislocations generally have line directions that are close to perpendicular to the local growth surface and that, as a result, the strain-related birefringence shows a characteristic anisotropy, being much more obvious for a viewing direction parallel to the growth direction.
WO2004/046427 A1 describes the production of “optical quality diamond material” via the CVD process by utilising a controlled, low level of nitrogen to control the development of the crystal defects. It is described how nitrogen present in the CVD diamond material must be sufficient to prevent or reduce local strain generating defects whilst being low enough to prevent or reduce deleterious absorptions and crystal quality degradation.
U.S. Pat. No. 6,096,129 describes a method of growing diamond material on a substrate surface such that the grown diamond material has a larger area than the starting substrate. The reference describes providing an initial single crystalline diamond base material, onto which single crystalline diamond material is homoepitaxially vapour deposited to provide a resulting diamond material that is cut and polished to provide a successive base material onto which single crystalline material is again grown, thereby forming a single crystalline diamond material having a large area. As best exemplified in FIGS. 4A-4C of U.S. Pat. No. 6,096,129, the initial base material is substantially square with {100} side surfaces, growth taking place predominantly on an upper {001} surface, that growth taking place laterally as well as normally from the upper {001} surface so that the grown surface has enlarged lateral dimensions compared to those of the initial base material. The successive base material that is cut from the grown diamond material is square in cross-section. The sides of the square are rotated 45° relative to the sides of the initial base material, and have <110> edges. The area of the square cross section of the successive base material is less than twice the area of the square cross-sectional area of the initial base material, due to the encroachment of {111} faces in the grown diamond material. This successive base material is then used for further growth, this further growth being from the <110> edges. The preferred growth rate ratio α is said to be at least 3:1.
The growth rate ratio, α, is a parameter that can be monitored in a CVD single crystal diamond material process, and is well understood in the art of diamond material synthesis by CVD. The parameter α is proportional to the ratio of the growth rate (GR) in the <001> direction (R<001>) to the growth rate in the <111> direction (R<111>), and is defined as:
  α  =                              3                ×                  R                      <            001            >                                      R                  <          111          >                      .  
In known CVD processes the α parameter is known to vary, typically over the range 1 to 3, the value of α depending, inter alia, on the set of synthesis conditions in place, including the pressure, the temperature and the gas flow conditions. The parameter α can be calculated after synthesis has been completed by making measurements on the as-grown diamond materials and using simple geometric relationships and crystallography to calculate α. It is also known in the art to make an ‘α parameter map’ of a particular synthesis process by measuring diamond materials grown under a range of pressure, temperature and gas composition combinations—again by post facto measurements. The methodology of characterising the α parameter for a given set of conditions is reported widely, however a particularly useful reference is Silva et al., Diamond & Related Materials (2009), doi:10.1016/j.diamond.2009.01.038. Silva et al describes how to select the temperature, gas pressure, power, and the process chemistry (e.g. the amount of methane, oxygen, nitrogen, hydrogen and argon gas etc) in order to achieve predetermined values of the α parameter. The exact values of each of these properties are specific to the process used by Silva, but the skilled man can readily characterise any other process, and select appropriate values for each of the above properties using the teaching of Silva et al in order to achieve the desired α parameter.
Single crystal diamond finds a potential application within Raman lasers, as described in US 2005/0163169. Such an application places severe requirements on the diamond material that can be utilised.
Raman lasers rely on the process of Raman scattering. Spontaneous Raman scattering occurs when a photon incident on a material results in the excitation of a vibrational mode from its initial energy level to an excited, virtual state. This virtual state can then return to an energy level different to the original level, producing a photon of different energy (and frequency) to that of the incident photon. For the majority of the spontaneously Raman scattered photons the final energy level is higher than the initial level, the scattered photon therefore has a lower energy than the incident photon, this is termed Stokes scattering. The energy difference between the incident and scattered photons results in the production of a phonon (a quantised lattice vibration).
In a Raman laser the scattered photon is utilised to stimulate further Raman scattered photons of the same wavelength: stimulated Raman scattering (SRS). This is achieved by feeding the scattered photon back into the Raman scattering medium, commonly by keeping the Raman scattering medium within an appropriate optical resonator, as described in Optics Express, 2008 16 (23), pages 18950-18955 and Optics Letters, 2009 34, pages 2811-2813.
In spontaneous Raman scattering it is also possible to observe second Stokes photons due to the Raman scattering of the first Stokes photon. This process can be repeated further such that a succession of higher order Stokes photons are observed with frequencies equal to the pump photon frequency minus an integer number of the characteristic phonon frequency. In a Raman laser, these higher order Stokes wavelengths can in principle be engineered to be the main emission wavelength of the device by simply designing the optical cavity to resonate at the desired Stokes wavelength.
The Raman laser is therefore capable of changing the frequency of the input light, advantageously producing an output beam with a frequency in a part of the electromagnetic spectrum that was previously unattainable with conventional laser technology.
Single crystal diamond is a promising material for use as the Raman scattering medium within the Raman laser. It has a high Raman gain coefficient, possesses low absorbance in a wide range of the electromagnetic spectrum (allowing versatility in the choice of input, intermediate and output frequencies), it is a good dissipater of thermal energy which is generated in the form of phonons as an integral part of the process, and possesses a low thermal expansion coefficient (minimising temperature related distortions).
The Raman gain coefficient, gR, is defined as
      g    R    =      const    ×                  T        2                    λ        S            where T2 is the optical phonon decoherence time, λS is the Stokes-shifted output wavelength and const is a material dependent constant of proportionality.
There are several considerations when optimising diamond material for use as the Raman scattering medium in a Raman laser. Point defects must be minimised in order to minimise absorption (and so potential efficiency reduction). Dislocations must be minimised in order to minimise birefringence (and so minimise detrimental effects when the material is utilised in polarisation sensitive applications). All defects must be low to maintain a high thermal conductivity so that the Raman scattering medium is able to handle high input powers while minimising temperature related distortion of the material. The material must also have a long internal path length for the incident light as this reduces the threshold of the laser device (the minimum input power required for the device to act as a laser).
In addition to the crystalline quality of the Raman scattering medium, the polarisation of the pump beam with respect to the symmetry axes of the crystal is another parameter which affects the Raman gain coefficient. For a linearly polarised pump beam with polarisation vector along a <110> direction, the Stokes beam is polarised parallel to the pump beam. For a pump beam polarised along a <100> direction the Stokes beam is polarised perpendicular to the pump beam. This suggests a particularly convenient crystallographic configuration of the diamond gain crystal, in which a rectangular block with two pairs of {110} faces and one pair of {100} faces is manufactured. By pumping the diamond Raman scattering medium crystal through a {110} face at the Brewster angle with p-polarised light, reflections at the incident and exit faces are eliminated for both the pump and Stokes beams. An even better configuration entails processing Brewster facets on the incident/exit faces, such that the pump beam within the crystal proceeds along the <110> direction. This then ensures that there is no component of the pump beam direction perpendicular to the length of the cavity and so there is also no risk of the pump beam exiting one side of the crystal and that the pump beam lies accurately on the <110> direction.
By engineering the diamond in such a way as to accommodate Brewster angle pumping, the requirement for anti-reflection coatings, which add cost and complication, is avoided.
Diamond Raman lasers can operate in a number of configurations. The most simple is as a Raman generator, in which a high intensity pulsed pump laser 2 is focused onto the diamond Raman gain crystal 4, resulting in conversion of the pump wavelength to multiple Stokes orders which constitute the output beam 6 of the laser (FIG. 1). Although this is a relatively simple design which does not require an optical cavity, in practice such a configuration is of little use due to the limited control of the output spectrum
A second type of configuration is as an external Raman resonator. Here the Raman crystal 4 is placed within an optical resonator comprising an input mirror 8 and an output mirror 10 in order to reduce the SRS threshold, increase the conversion efficiency and tailor the output wavelength 14 (FIG. 2). In this configuration the cavity is pumped externally with either a continuous wave (cw) or pulsed laser source 12. Due to diamond's high Raman gain coefficient the Raman crystal can be kept short compared to other Raman gain materials. Such an external diamond Raman resonator can therefore be viewed as a simple, compact add-on enabling frequency conversion for a wide variety of laser sources.
A third configuration is an intracavity Raman resonator, in which both the pump laser medium 16 and the Raman crystal 4 are placed within a cavity, comprising input mirror 8 and output mirror 10, resonant at both pump and Stokes wavelengths (FIG. 3). This configuration takes advantage of the high intracavity pump field which leads to enhanced conversion to the output beam 20. The cavity may also include other optical elements such as a Q-switch 18 for pulsed mode operation.
There is therefore a need to produce a diamond material with fewer point defects and fewer dislocations while maintaining a long internal dimension.